1. Field of the Invention
The present invention relates to methods and apparatus for evaluating the power dissipation capabilities of solid-state circuit elements, and in particular to methods and apparatus for the determination of the junction-to-case thermal resistance, .theta..sub.jc, of hybrid circuit elements.
2. Description of the Technology
An integrated circuit is normally encapsulated inside a plastic or metal package that has leads which can be soldered to a printed-circuit board or plugged into a connector. The working part of each package is only a small part of the volume, however. If each tiny integrated circuit is attached directly to a substrate rather than placed inside an individual package, several integrated circuits can be interconnected in circuits within a single package, thus eliminating the need for the printed-circuit board. Resistors can be fabricated on the substrate, and capacitors and discrete semiconductor elements can also be added, so that a completely self-contained interconnected circuit called a chip "hybrid" is formed.
A single hybrid can serve as a small, lightweight substitute for a printed-circuit board or collection of printed circuit boards, with the added benefit of considerably increased reliability. Hybrids have been widely used in aerospace and military applications. Other uses have been confined mostly to products produced in large volume, for example digital-to-analog converters and microwave communication circuits.
Hybrids are used for their increased reliability in applications such as cardiac pacemakers, telecommunications, automotive braking systems, and computer circuits. Hybrid reliability is generally regarded to be about ten times greater than that of an equivalent discrete circuit board.
The reliability of discrete components mounted on circuit boards suffers primarily from two failure mechanisms. First, the solder connections on the board can fail from from shock and vibration. Second, the semiconductor components are only lot-sample tested by manufacturers, so that some small number of faulty components are sent to any customer. Lot-samples are operated under load for only fractions of a second because more elaborate testing increases manufacturing costs prohibitively. Minimal tests are not good approximations of the conditions encountered by the components in service, however, and the user must therefore test each component to ensure reliability.
Hybrid interconnection methods reduce these problems. To eliminate solder connections, some monolithic hybrids have components formed within the substrate as in a large-scale integration chip. More often, components are chemically attached to the substrate. Chemical bonding methods are more reliable than making solder connections. Solder connections are still used on some hybrids, but far fewer are required than with discrete components on circuit boards. Most hybrids are encapsulated in epoxy, which further reduces the effects of mechanical vibration. The lot-sample testing problem with discrete integrated circuits is overcome by the fact that every hybrid comes in a complete package and can be readily tested.
The junction-to-case thermal resistance, .theta..sub.jc, of a solid-state circuit element is of critical importance in circuit design and circuit component selection. A knowledge of its value is necessary for heat transfer calculations in the design of heat sinks to dissipate the power developed in a solid-state circuit element. Inadequate heat sinks designed on the basis of wrong values of .theta..sub.jc can result in disastrous consequences if crucial circuit elements fail in vital control and other types of circuits.
Previous methods of determining .theta..sub.jc, have used direct current approaches that reguire repetitive testing followed by averaging of data. Such methods are time-consuming, inconvenient, and inefficient. There has been a long felt but unfulfilled need for a convenient yet quick and accurate method to determine the junction-to-case thermal resistance of hybrid solid-state circuit elements. Such a method would be a boon to the electronics industry and the large number of associated industries that require reliable electronic components for their products and for their manufacturing and test equipment.